fix: TLA in web worker (#8809)

Implementors of `deno_core::JsRuntime` might want to do additional actions
during each turn of event loop, eg. `deno_runtime::Worker` polls inspector,
`deno_runtime::WebWorker` receives/dispatches messages from/to worker host.
Previously `JsRuntime::mod_evaluate` was implemented in such fashion that it 
only polled `JsRuntime`'s event loop. This behavior turned out to be wrong 
in the example of `WebWorker` which couldn't receive/dispatch messages because
its implementation of event loop was never called. 

This commit rewrites "mod_evaluate" to return a handle to receiver that resolves
when module's promise resolves. It is now implementors responsibility to poll
event loop after calling `mod_evaluate`. 

2 files changed, 34 insertions, 0 deletions

diff --git a/cli/tests/worker_with_top_level_await.ts b/cli/tests/worker_with_top_level_await.ts
new file mode 100644
index 000000000..cf3418bf7
--- /dev/null
+++ b/cli/tests/worker_with_top_level_await.ts
@@ -0,0 +1,15 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+import { serve } from "../../std/http/server.ts";
+
+const server = serve({ port: 8080 });
+
+self.onmessage = (e: MessageEvent) => {
+  console.log("TLA worker received message", e.data);
+};
+
+self.postMessage("hello");
+
+for await (const _r of server) {
+  // pass
+}
diff --git a/cli/tests/workers_test.ts b/cli/tests/workers_test.ts
index d907c97a9..4f7682be2 100644
--- a/cli/tests/workers_test.ts
+++ b/cli/tests/workers_test.ts
@@ -357,3 +357,22 @@ Deno.test({
     w.terminate();
   },
 });
+
+Deno.test({
+  name: "Worker with top-level-await",
+  fn: async function (): Promise<void> {
+    const promise = deferred();
+    const worker = new Worker(
+      new URL("./worker_with_top_level_await.ts", import.meta.url).href,
+      { deno: true, type: "module" },
+    );
+    worker.onmessage = (e): void => {
+      console.log("received from worker", e.data);
+      worker.postMessage("from main");
+      promise.resolve();
+    };
+
+    await promise;
+    worker.terminate();
+  },
+});